1. Field of the Invention
This invention relates generally to nuclear reactors and, more particularly, to a method and apparatus for clamping a riser brace assembly to a jet pump assembly of a boiling water reactor.
2. Description of Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1 is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV) 20 for a boiling water reactor. The RPV 20 has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate 22 within RPV 20. A shroud 24 surrounds core plate 22 and is supported by a shroud support structure 26. A downcomer annulus 28 is formed between shroud 24 and sidewall 30 of RPV 20.
An annulet nozzle 32 extends through sidewall 30 of RPV 20 and is coupled to a jet pump assembly 34. Jet pump assembly 34 may include a thermal sleeve 36 which extends through nozzle 32, a lower elbow (only partially visible in FIG. 1), and a riser pipe 38. Thermal sleeve 36 is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve 36 is welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe 38. Riser pipe 38 extends between and substantially parallel to shroud 24 and sidewall 30.
A riser brace assembly 40 stabilizes a riser pipe 38 within the RPV 20. The riser brace assembly 40 may be fabricated of type 304 stainless steel which, after periods of use, is susceptible to cracking at welded joints. The riser brace assembly 40 is fixedly connected between shroud 24 and sidewall 30, and primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, as shown in FIG. 1. Additionally the riser brace assembly 40 is designed to accommodate for differential thermal expansion that results from reactor start-up and heat-up, and flow induced vibration that is incumbent in the reactor water recirculation system (not shown).
FIG. 2 illustrates a riser brace assembly 40 of FIG. 1. The riser brace assembly 40 primarily provides lateral support to the jet pump assembly 34 via riser pipe 38, and includes a riser brace block 43 and two riser brace leaves, an upper riser brace leaf 41 and a lower riser brace leaf 42. Leaves 41 and 42 are attached to the riser brace block 43 by welds, and the riser brace block 43 is welded to a support pad 130 which in turn is attached to a RPV sidewall 30. At the other end, the riser brace assembly 40 is connected to a yoke, such as brace plate 49, which is typically a ½-inch thick plate that is welded to the riser pipe 38.
In the riser brace assembly 40 of FIG. 2, there may be numerous weld sites including welds that attach the riser brace plate 49 to riser pipe 38, welds attaching the riser brace block 43 to the support pad 130, and welds attaching the leaves 41 and 42 to the brace plate 49. These welds are typically field welds (made on site). The welds connecting riser brace block 43 to upper and lower riser brace leaves 41 and 42 are shop welds (e.g., pre-fabricated in the shop).
FIGS. 3A and 3B illustrate another riser brace assembly 40. This riser brace assembly 40 also provides lateral support to the jet pump assembly via riser pipe 38, and includes a riser brace support 49 and two riser brace leaves, an upper riser brace leaf 41 and a lower riser brace leaf 42. However, in this assembly, the riser brace support 49 is welded to the riser pipe 38 at two weld sites. The two welds attaching the riser brace support 49 to the riser pipe 38 are designated as RS-8 and RS-9, as shown in cross section A-A in FIG. 3B. In addition, the welds attaching the riser brace leaves 41, 42 to the riser brace support 49 are indicated as RB-2a and RB-2c. It should be understood that only two weld points RB-2a, RB-2c are shown in this figure since welds RB-2b and RB-2d (not shown) are hidden in the figure.
However, lack of weld integrity will lead to failure of the riser brace assembly which provides support to the jet pump assembly. For instance, weld failure due to vibration fatigue, and/or weld cracking due to intergranular stress corrosion cracking (IGSCC) could cause one of the welds joining the riser brace assembly 40 to the RPV 20 to fail. As an illustrative point, FIG. 3A illustrates a cracked weld RS-9 between the riser brace assembly 40 and the riser pipe 38. Separation of the riser brace assembly 40 near this or any other weld area could adversely impact safety in BWRs. Potentially, should a riser brace assembly 40 break away from RPV 20 (e.g., at RPV sidewall 30) and/or the riser pipe 38, the riser pipe 38 may become unstable, and the jet pump assembly 34 could be adversely affected. If just one jet pump assembly is damaged, a substantial amount of piping must either be replaced or repaired. Since weld repairs in the area of the downcomer annulus 28 are typically not practical due to inaccessibility, and the potential for excessive radiation exposure to personnel is real, a need exists for a method and apparatus of securely clamping the riser braces to the jet pump assembly.